In a video sequence, composed of successive interlaced pictures (or frames), each frame is constituted by a pair of fields F1 and F2, as illustrated in FIG. 1 showing successive pairs of fields (each frame comprises a top field F(2n−1) (with n>0), or odd field, and a bottom field F(2n), or even field, the odd frames being of type F1 and the even frames of type F2) and the associated synchronization signal. When such video fields come out, for instance at a rate of 50 fields/second (25 frames/second) or 60 fields/second (30 frames/second), either of a video camera or of any other type of video signal generator, the video material has no field dominance (a frame is said to be “F1 dominant” if it is constituted by a first field F1 followed by a second field F2, and to be “F2 dominant” if it is constituted by a field F2 followed by a field F1).
The field dominance becomes relevant when transferring data in such a way that frame boundaries must be known and preserved. When the video material is edited at frame boundaries, with a video recorder for example, a decision is provided for specifying if the video material is F1 dominant or F2 dominant: FIGS. 3 and 4 respectively show, for a preexisting video material as indicated in FIG. 2, the structure of a F1 dominant video material and of a F2 dominant video material. Once some material has acquired a particular chrominance, it must be manipulated with that dominance. Otherwise, a shift can occur in the representation of a frame, as shown in FIG. 5: the two first frames are F1 dominant, but the third one is F2 dominant and composed of two fields which originally did not belong to the same frame. In such a case, encoding is less efficient: a scene cut between the two fields of an encoded frame costs a lot in terms of bitrate allocation efficiency. Moreover, F2 dominance may lead to annoying vertical moving of pictures when a DVD player outputs frames in slow motion or still image mode.